Field of the Invention
The invention relates to an optical resonator made of low-outgassing materials, comprised of at least one chamber, a non-linear crystal arranged in the chamber, and an array of mirrors arranged in the chamber and comprised of a plurality of mirrors for deflecting a light beam.
Related Art
Optical resonators of this type, for example, are utilized for frequency conversion of a light beam, more particularly a laser beam. Requirements exacted from the setup of an optical resonator are high. In case of generating ultraviolet laser radiation, in particular, high purity in the surrounding of the non-linear crystal is required. What is striven for is an environment composed of low-outgassing materials, whereby the choice of materials eligible for the setup of a resonator is noticeably confined.
The term “outgassing” defines a process in which materials dissipate impurities even down to extremely low partial pressures (<10−10 Torr). This liberation of gases (outgassing) is problematic for an optical resonator, because impurities settle down on optical elements—e.g. on mirrors and on the non-linear crystal—and thus may adversely affect the functional mode, more particularly the quality of the optical resonator. Eligible as low-outgassing materials, for example, are metals and glasses, but even PTFE and piezoelectric ceramics and crystals.
To date, excimer and solid state laser systems are applied in the ultraviolet radiation range. In operation, they usually emit at a fixed wavelength. In this case, the resonators can be adjusted ex works, thus not calling for any (post) adjustment of the optical elements if wavelength varies.
The situation is different with diode lasers, whose wavelength can be varied over a greater spectral range. In this case, the optical elements and the non-linear crystal must be movable in order to be able to consider a shift in wavelength. The non-linear crystal, in particular, must be rotatable about the correct phase alignment angle in case of a wavelength shift and be adjustable, unless other phase alignment mechanisms as for example via temperature are insufficient.
On account of the limited choice of low-outgassing materials on the one hand and on the other hand because of the requirement demanding configuring the resonator in such a manner that a fine adjustment of the optical elements with regard to wavelength alignment is feasible, in particular when using light within the ultraviolet range, those optical resonators currently available do not fulfill all requirements exacted and hence they should be regarded as a compromise solution in which either the selected materials are low-outgassing materials or in which a fine adjustment of the optical elements is feasible.